In the process of coal gasification, coal is converted to volatile, gaseous products which are useful as sources of energy. These gaseous products usually include significant amounts of sulfur compounds such as hydrogen sulfide. These sulfur compounds are pollutants and must be removed from the product gas in some manner.
A current approach to hot gas desulfurization in fluid-bed coal gasifiers for gas turbine and fuel cell application is to combine in-bed desulfurization using lime/limestone with an external desulfurization process employing a fixed bed of zinc ferrite or an equivalent sulfur adsorbing compound. Generally, up to about 80% of the coal sulfur is removed by the lime/limestone and most of the remaining sulfur is removed by the external zinc ferrite fixed bed. In this process, the gas exiting the gasifier is partially quenched from about 1600.degree. to 1800.degree. F. to about 1000.degree. to 1200.degree. F. with water. This quenching ensures that the zinc ferrite sorbent remains stable and zinc vapor formation from the zinc ferrite bed is within acceptable limits.
The external desulfurization step employing a fixed bed of zinc ferrite is costly. Thus, there is a cost incentive to increase the amount of desulfurization accomplished within the gasifier. However, increased desulfurization within the gasifier cannot be accomplished by increasing the calcium to sulfur ratio within the gasifier bed because of chemical reaction equilibrium constraints. Thus, it is desirable to develop some other method for increasing desulfurization within the gasifier.